The rotary hydraulic motor shown in prior U.S. Pat. No. 2,795,212 has cross-over ports through the rotor shaft connecting the opposite fluid chambers and a pressure or exhaust port extending through the side wall connected with one of each pair of opposite fluid chambers. The pressure and exhaust ports are connected with the bottoms of tanks containing oil, and gas pressure is applied to the top of one tank while the top of the other is connected to exhaust, as shown in prior U.S. Pat. No. 2,915,042. The rotor shaft is connected to the valve stem of a rotary valve in a pipeline, and fluid flow in one direction closes the valve while reversing the flow opens it.
The continuous oil system from one tank through the hydraulic motor to the other tank provides a non-compressible fluid which substantially eliminates cushioning of the rotor piston which would result in floating or overtravel when stopped. However, due to the high pressure of the gas applied to the oil tanks some gas bubbles are entrained in the oil.
The operation of drilling the cross-over ports in the rotor shaft was difficult and expensive, and in the hydraulic motor in prior U.S. Pat. No. 2,811,142, the cross-over ports comprised annular grooves around the rotor shaft in the top and bottom plates of the motor, one groove connecting opposite chambers of one pair and the other groove connecting opposite chambers of the other pair. As the rotor shaft is always disposed vertically the entrained gas bubbles rose to the top and accumulated in the upper cross-over groove, where they remained trapped despite reversing the flow through the motor because the pressure and exhaust ports were in the side wall of the motor cylinder. This accumulated gas usually contains acid and causes corrosion, as well as causing cushioning of the rotor.